Ways have been described for patterning wiring circuit elements having metal wiring patterns exposed on one side and a plurality of posts or bumps extending from another side opposite to the one side. Typically, the first metal wiring patterns are connected vertically by the posts with second wiring patterns to which the posts are joined later. Usually, a dielectric layer is provided overlying the first metal wiring patterns as an insulating layer between the first and second wiring patterns.
One known way of fabricating the first metal patterns and conductively connected posts is to pattern them from a layered metal structure that includes an inner layer consisting essentially of nickel sandwiched, i.e., disposed between first and second layers of copper. The nickel layer functions as an etch stop layer when the first and second copper layers are patterned, such that each of the first and second copper layers can be patterned in accordance with different masking layers.
However, there are problems associated with this method. One problem is that nickel is attacked by cupric chloride (CuCl2), an etchant which is commonly used for patterning features from a copper layer. In order to address this problem, an ammoniacal etchant can be used to pattern the copper layers. Sometimes, a cupric chloride containing etchant is used to pattern a portion of the thickness of a copper layer, when contact between the etchant and the nickel layer can be avoided. Subsequently, an ammoniacal etchant is applied to finish etching through the thickness of the copper layer, stopping on the nickel etch stop layer.
Ammoniacal etchants are not as not as commonly used as cupric chloride, and processes using such etchants typically are more difficult to control than those in which cupric chloride is used.
Another concern is that nickel forms a relatively porous layer when deposited, whether deposited by vapor deposition, e.g., sputtering or chemical vapor deposition, or by electroplating. Because of this, the nickel layer needs to be sufficiently thick to avoid the emergence of pinholes which extend through the thickness of the nickel layer. Typically, the thickness of a nickel layer provided as an etch stop layer between first and second copper layers is about 5 microns. The relatively large thickness of the nickel layer requires that a corresponding etchant quantity and time need to be expended during subsequent processing to remove unwanted portions of the nickel layer after the copper layers have been patterned.